The main problem area in a close-solar-approach mission is temperature. The problem begins with the fact that there is more than sufficient light to generate the power required and the unused heat input to the solar cell from the sun limits the closeness of the approach to the sun. At about a value of 0.1 AU, for example, the heat input to the solar cells is about 14 W-cm.sup.-.sup.2. At this input level, the cell temperature rises above 400.degree.C without special cooling. Because 200.degree.C is the maximum operating temperature of the silicon solar cell, special steps must be taken to lower the temperature. The heating problem is also aggravated by internal (series) resistance losses in the cell. The standard cell is tailored to operate at light intensities as high as 0.28 W-cm.sup.-.sup.2 without suffering excessive losses due to internal resistance (caused primarily by the tin, diffused surface layer). However, as the intensity rises, part of the usable power is dissipated in this resistance and this contributes not only to the heating problem but also to an undesirable loss in power output. Therefore, in order to eliminate this contribution to the heating problem, the series resistance must be reduced or the solar intensity reaching the cell must be kept below 0.28 W-cm.sup.-.sup.2.
Several different approaches have been taken in an attempt to solve the temperature problem discussed above. First, it has been proposed that the solar cell array be tilted at an angle to the sun to reduce the power input. However, this approach is limited in its applicability because there could arise undesirable reflections that could cause heating of the spacecraft. Also, additional control maneuvers and altitude control measures on the solar cell array or the spacecraft would be required and those can complicate placing of the scientific sensors and detectors on board the spacecraft. The angle of tilt is also limited from practical considerations of alignment, stability and control.
Next, the array can be cooled with extra radiating surfaces. These can be attached to the back of the array, but at the cost of adding undesirable weight. Another approach that has been used is to increase the panel area, with only partial coverage by the solar cells, the remaining area on the front being covered with mirrors. The mirrors would be used to prevent solar radiation from being absorbed in those regions thus increasing the effective radiating area of the array. This approach, however, causes undesirable temperature gradients across the array as well as adding undesirable weight.
Further, the solar cells themselves have been coated with totally reflective stripes or patterns. The problems in this approach are similar to those encountered in the large area panel approach except the scale has been changed. The cell will have undesirable temperature gradients across its surface and the local areas that are illuminated will be inefficient due to the internal resistance problems described earlier and will thus contribute to the heating problem.
Finally, a further prior art approach of particular interest here is that disclosed in U.S. Pat. No. 3,532,551 (Scott). The Scott patent discloses a solar cell to which a fused silica cover plate is bonded, with portions of the cover plate overhanging the edges of the solar cell. A reflective coating is deposited on the lower surface of the overhanging portions of the cover plate to reduce the amount of heat absorbed by the cell and hence reduce the overall cell temperature. The advantages of the present invention over the technique taught in the Scott patent are set forth below.
The present invention is also applicable to the area of monitoring high intensity light sources. Although these sources can be monitored with thermopiles, the response such thermopiles provide is slow and does not provide a real-time measurement of the source variation. A bare solar cell used for this purpose suffers from the problems of heat and series resistance described previously. The heat problem is acute and can be partially solved by use of an auxiliary cooling system. However, this is an undesirable approach because it adds additional complexity, the cooling system requiring a bulky cooling block, a coolant circulating arrangement and suitable controls. Heat transfer through the cell to the cooling block is critical and cell mounting presents substantial problems. Further, the series resistance of the cell under high illumination is also a problem and can easily lead to errors in measurement if the cell short-circuit current is being taken as the index of intensity. Interference filters, such as formed by multilayer dielectrics, can be used to reflect most of the light away except for a particular wavelength interval. However, there are several problems in this approach also. The filters are expensive and are prone to heating up, to changing transmission characteristics and to even cracking or other damage. As described below, the present invention can be used to circumvent all these problems and provide a rapid, accurate, real-time sensor for monitoring high intensity light sources.